1. Field of the Invention
This invention relates to the manufacture of glass ribbon by the flotation process.
2. Description of the Prior Art
In the flotation process for manufacturing glass ribbon, a thick layer of molten glass is poured onto a bath of molten metal. Because the density of the molten glass is less than that of the molten metal, it floats upon the molten metal bath. The glass is caused to advance along the length of the molten metal bath and is also permitted to flow freely in a lateral direction so that the thickness of the advancing molten glass is gradually reduced. The molten glass naturally acquires a thickness of approximately 6 mm due to the effect of gravitational forces and of the interfacial tension between the glass layer and the molten metal bath. The naturally acquired thickness of the glass ribbon is customarily described as its equilibrium thickness.
As the molten glass advances along the length of the molten metal bath, the glass is gradually cooled to form a flat, congealed glass ribbon floating upon the molten metal bath. Knurled extraction rollers facilitate the forward motion of the glass ribbon and draw the relatively cool glass ribbon from the molten metal bath. The extraction rollers also assist in the manufacture of glass ribbon having a thickness less than the equilibrium thickness. By adjusting the rate of rotation of the extraction rollers, traction forces exerted by the rollers upon the glass ribbon may be increased so as to stretch the glass ribbon to a thickness of approximately 2 mm.
With the flotation process, it is also possible to manufacture glass ribbon having a thickness greater than the equilibrium thickness. Molten glass is poured onto the molten metal bath but the molten glass is physically prevented from outwardly spreading to its equilibrium thickness. When the molten glass has reached a desired thickness, its lateral spreading is halted by a series of barriers positioned within the molten metal bath along both sides of the advancing glass ribbon. The edges of the continuously advancing ribbon of glass slide along these barriers. To reduce the adherence of the molten glass to the barriers, a non-wettable material such as carbon or graphite is used to form the barriers. By preventing the lateral spreading of the molten glass as well as regulating the rate of addition of molten glass to the molten metal bath and the rate of advance of the glass ribbon, it is possible to manufacture glass ribbon having a thickness ranging from approximately 7 to 20 mm.
As the hot molten glass is poured into the molten metal bath, heat is transferred from the molten glass to the bath; and the central region of the bath tends to become considerably hotter than the sides of the bath. It is desirable to reduce the thermal gradient thus formed across the molten metal bath to ensure that the glass ribbon floating upon the molten metal bath has flat, parallel surfaces. To equalize the temperature across the molten metal bath and to minimize the effect of processing asymmetries, it is known to create transverse convection currents of molten metal beneath the glass ribbon and to direct these currents toward the edges of the glass ribbon, thereby producing a thermosiphon.
In the prior art, non-wettable cooling members positioned within the molten metal bath on either side of the edges of the glass ribbon are used to create the above described transverse convection currents. Since the local transverse flow of molten metal drives the glass ribbon to the corresponding side of the molten metal bath, the cooling members must also serve as thrust bearings which exert a lateral thrust on the edge of the glass ribbon which contacts the cooling member.
As indicated in French Pat. No. 1,535,007, the cooling members may be cooled internally by the circulation of a cooling fluid. This internal circulation assists in preventing the edges of the glass ribbon from adhering to the contacted cooling member.
Th prior art cooling members have several limitations. For example, since these internally cooled members are immersed within the molten metal bath, they substantially cool the side areas of the bath. As previously indicated, these side regions have a natural tendency to be cooler than the central region of the bath. The thermal gradient across the molten metal bath is thus increased by the prior art cooling members. As a result, the glass ribbon will not have flat parallel surfaces. The increased thermal gradient also enhances the undesirable tendency of the glass ribbon to divert toward the relatively cold side regions of the molten metal bath and contact the cooling members.
Although the lower surface of the prior art cooling members may be thermally insulated from the bath of molten metal, the bath is substantially cooled by even the small uninsulated portion of the side walls of the cooling members which extend into the molten metal bath.